JP6319443B2 - Steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material and a method for producing the same, and more particularly to a steel material having a tensile strength of 980 MPa or more and having excellent ductility and impact characteristics and a method for producing the same.

  In recent years, from the viewpoint of protecting the global environment, development of steel materials that contribute to energy saving has been demanded. In the fields of automobile steel, oil well pipe steel, and building structural steel, demand for ultra-high strength steel that can be applied to light and harsh usage environments is increasing, and its application range is widening. As a result, in ultra-high strength steel materials used in these fields, it is important to ensure not only strength characteristics but also safety in the usage environment. Specifically, it is important to increase the tolerance for external plastic deformation by increasing the ductility of the steel material.

  For example, when an automobile collides with a structural body, the tensile strength of the steel material is 980 MPa or more, and the tensile strength (TS) and total elongation (EL ) And the product value (TS × EL) is preferably 16000 MPa ·% or more. However, since the ductility is remarkably lowered as the tensile strength is increased, there has been no ultrahigh strength steel material that satisfies the above characteristics and can be industrially mass-produced. Therefore, various research and development have been made in order to improve the ductility of the ultra-high strength steel material, and a structure control method has been proposed (see Patent Documents 1 to 4).

  However, with the conventional technology, sufficient ductility and impact characteristics cannot be obtained while securing a tensile strength of 980 MPa or more.

JP 2004-269920 A JP 2010-90475 A JP 2003-138345 A JP 2014-25091 A

  An object of the present invention is to provide a steel material having excellent ductility and impact properties while having a tensile strength of 980 MPa or more, and a method for producing the same.

  The present inventors have intensively studied to solve the above problems. As a result, the following knowledge was obtained.

  When a steel material is heated to a two-phase region of ferrite and austenite, the surface is decarburized to form a structure composed of a soft ferrite phase (hereinafter referred to as “decarburized ferrite layer”). And if decarburization becomes remarkable, a decarburized ferrite layer will be formed thickly on the surface of steel materials.

  When the thickness of the decarburized ferrite layer is 5 μm or more, coarse ferrite is generated, and as a result, ductility and impact characteristics may be deteriorated.

  Therefore, in order to produce a high-strength steel material, an appropriate heat treatment was performed on a steel material that contains Si and Mn more actively than usual to suppress decarburization on the surface. As a result, it has been clarified that a steel material having excellent ductility and impact properties can be stably obtained while having a tensile strength of 980 MPa or more, which could not be produced by conventional techniques.

  The present invention has been made on the basis of the above findings, and the gist of the present invention is the following steel materials and manufacturing methods thereof.

(1) In mass%,
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: more than 3.0% and 7.5% or less,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%
Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1.0%
Mo: 0% to 1.0%
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0.01%
Zr: 0% to 0.01%,
B: 0% to 0.01%
Bi: 0% to 0.01%, and the balance: Fe and impurities,
Having a chemical composition represented by
The decarburized ferrite layer has a metal structure having a thickness of 5 μm or less, a volume ratio of retained austenite of 10% to 40%, and a number density of cementite of less than 2 pieces / μm ² ,
A steel material having a tensile strength of 980 MPa or more.

(2) In the chemical composition,
V: 0.05% to 1.0%
Is satisfied. The steel material as described in said (1) characterized by the above-mentioned.

(3) In the chemical composition,
Ti: 0.003% to 1.0%,
Nb: 0.003% to 1.0%,
Cr: 0.01% to 1.0%
Mo: 0.01% to 1.0%
Cu: 0.01% to 1.0%, or Ni: 0.01% to 1.0%,
Or any combination of these is satisfy | filled, The steel materials as described in said (1) or (2) characterized by the above-mentioned.

(4) In the chemical composition,
Ca: 0.0003% to 0.01%,
Mg: 0.0003% to 0.01%
REM: 0.0003% to 0.01%,
Zr: 0.0003% to 0.01%,
B: 0.0003% to 0.01%, or Bi: 0.0003% to 0.01%,
Or any combination of these is satisfy | filled, The steel materials in any one of said (1) to (3) characterized by the above-mentioned.

(5) the average C content in the residual austenite, in mass%, the steel of any one of (1), characterized in that at 0.60% of (4).

(6) The method for producing a steel material according to any one of (1) to (5) above,
% By mass
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: more than 3.0% and 7.5% or less,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%
Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1.0%
Mo: 0% to 1.0%
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0.01%
Zr: 0% to 0.01%,
B: 0% to 0.01%
Bi: 0% to 0.01%, and the balance: Fe and impurities,
Steel having a metal structure in which the volume ratio of bainite and martensite is 90% or more in total, and the average aspect ratio of bainite and martensite is 1.5 or more. Heating the material to a temperature of 670 ° C. or higher so that the average heating rate between 500 ° C. and 670 ° C. is 1 ° C./s to 5 ° C./s;
After the heating, holding for 60 s to 1200 s in a temperature range of 670 ° C. to 780 ° C.,
And a step of cooling to a temperature of 150 ° C. or lower so that an average cooling rate between the temperature range and 150 ° C. is 5 ° C./s to 500 ° C./s after the holding. Manufacturing method.

(7) In the chemical composition,
V: 0.05% to 1.0%
Is satisfied,
70% or more of V contained in the steel material is solid-dissolved, The method for producing a steel material according to (6) above, wherein

  According to the present invention, since the chemical composition and the metal composition are appropriate, a tensile strength with a tensile strength of 980 MPa or more and excellent ductility and impact properties can be obtained.

1. Chemical Composition First, the chemical composition of the steel material according to the embodiment of the present invention and the steel material used for manufacturing the steel material will be described. In the following description, “%”, which is a unit of content of each element contained in a steel material and a steel plate used for manufacturing the same, means “mass%” unless otherwise specified. The steel materials according to this embodiment and the steel materials used for the production thereof are C: 0.050% to 0.35%, Si: 0.50% to 3.0%, Mn: more than 3.0% and 7%. .5% or less, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.001% to 3.0%, N: 0.01% or less, V: 0% to 1.0%, Ti: 0% to 1.0%, Nb: 0% to 1.0%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 0.01%, Zr: 0% to 0.01%, B: 0% to 0.01%, Bi: 0% to 0.01%, And remainder: having a chemical composition represented by Fe and impurities. Examples of the impurities include those contained in raw materials such as ore and scrap and those contained in the manufacturing process.

C: 0.050% to 0.35%
C is an element that contributes to strength increase and ductility improvement. In order to obtain a steel material having a tensile strength of 980 MPa or more and a product of the tensile strength (TS) and the total elongation (EL) (TS × EL) of 16000 MPa ·% or more, the C content is set to 0.1%. It is necessary to set it to 050% or more. However, when C is contained exceeding 0.35%, the impact characteristics deteriorate. For this reason, C content needs to be 0.35% or less, and it is preferable to set it as 0.25% or less. In order to obtain a tensile strength of 1000 MPa or more, the C content is preferably 0.080% or more.

Si: 0.50% to 3.0%
Si is an element that contributes to the improvement of ductility by increasing the strength and promoting the generation of austenite. In order to make the product (TS × EL) value 16000 MPa ·% or more, the Si content needs to be 0.50% or more. However, when Si is contained exceeding 3.0%, the impact characteristics deteriorate. For this reason, Si content shall be 3.0% or less. In addition, in order to improve weldability, it is preferable that Si content shall be 1.0% or more.

Mn: More than 3.0% and 7.5% or less Mn, like Si, is an element that contributes to the improvement of ductility by promoting the generation of austenite with increasing strength. In order to make the steel material have a tensile strength of 980 MPa or more and a product (TS × EL) value of 16000 MPa ·% or more, it is necessary to contain Mn exceeding 3.0%. However, if Mn is contained exceeding 7.5%, refining and casting in the converter becomes extremely difficult. For this reason, the Mn content needs to be 7.5% or less, and preferably 6.5% or less. In order to obtain a tensile strength of 1000 MPa or more, the Mn content is preferably 4.0% or more.

P: 0.05% or less P is an element contained as an impurity, but may be positively contained because it is also an element contributing to an increase in strength. However, when P exceeds 0.05%, the weldability is remarkably deteriorated. Therefore, the P content is 0.05% or less. The P content is preferably 0.02% or less. When it is desired to obtain the above effect, the P content is preferably 0.005% or more.

S: 0.01% or less Since S is inevitably contained as an impurity, the lower the S content, the better. In particular, when the S content exceeds 0.01%, the weldability is remarkably deteriorated. For this reason, S content shall be 0.01% or less. The S content is preferably 0.005% or less, and more preferably 0.0015% or less.

sol. Al: 0.001% to 3.0%
Al is an element having an action of deoxidizing steel. In order to make steel materials sound, sol. Al is contained in an amount of 0.001% or more. On the other hand, sol. If the Al content exceeds 3.0%, casting becomes extremely difficult. For this reason, sol. The Al content is 3.0% or less. sol. The Al content is preferably 0.010% or more, and preferably 1.2% or less. Note that sol. Al content means content of acid-soluble Al in steel materials.

N: 0.01% or less Since N is inevitably contained as an impurity, the lower the N content, the better. In particular, when the N content exceeds 0.01%, the aging resistance is remarkably deteriorated. For this reason, N content shall be 0.01% or less. The N content is preferably 0.006% or less, and more preferably 0.004% or less.

  V, Ti, Nb, Cr, Mo, Ni, Ca, Mg, REM, Zr, and Bi are not essential elements, but are appropriately limited to a predetermined amount in the steel material according to the present embodiment and the steel material used for manufacturing the steel material. It is an optional element that may be contained.

V: 0% to 1.0%
V is an element that significantly increases the yield strength of the steel and prevents decarburization. Therefore, V may be contained. However, when V is contained exceeding 1.0%, hot working becomes extremely difficult. For this reason, V content shall be 1.0% or less. Moreover, in order to make the yield strength of steel materials 900 or more MPa, it is preferable to contain V 0.05% or more. In order to obtain a tensile strength of 1100 MPa or more, the V content is more preferably 0.15% or more. Moreover, when V is contained in the steel material, it becomes easy to adjust the average value of the aspect ratio of bainite and martensite to 1.5 or more in the steel material.

Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1.0%
Mo: 0% to 1.0%
Cu: 0% to 1.0%
Ni: 0% to 1.0%
These elements are effective elements for stably securing the strength of the steel material. Therefore, you may contain 1 or more types selected from said element. However, when both are included exceeding 1.0%, hot working becomes difficult. For this reason, the content of each element needs to be 1% or less. To obtain the above effect, Ti: 0.003% or more, Nb: 0.003% or more, Cr: 0.01% or more, Mo: 0.01% or more, Cu: 0.01% or more, or Ni: 0.01% or more, or any combination thereof is preferably satisfied. In addition, when 2 or more types of said elements are contained complexly, it is preferable that the total content shall be 3% or less.

Ca: 0% to 0.01%
Mg: 0% to 0.01%
REM: 0% to 0.01%
Zr: 0% to 0.01%
B: 0% to 0.01%
Bi: 0% to 0.01%
These elements are elements having an effect of increasing the low temperature toughness. Therefore, you may contain 1 or more types selected from said element. However, if the content exceeds 0.01%, the surface properties deteriorate. For this reason, the content of each element needs to be 0.01% or less. When it is desired to obtain the above effect, the content of one or more selected from these elements is preferably 0.0003% or more. In addition, when 2 or more types of said elements are contained complexly, it is preferable that the total content shall be 0.05% or less. Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

2. Metal structure Thickness of decarburized ferrite layer: 5 μm or less As described above, the decarburized ferrite layer is a structure composed of a soft ferrite phase formed by decarburizing the surface of a steel material during heat treatment. . Further, the decarburized ferrite layer is a structure including 90% or more of a ferrite phase having a columnar shape or a polygonal shape in an area ratio. In order to maintain excellent impact characteristics while having a high tensile strength of 980 MPa or more, it is necessary to suppress decarburization in the surface layer portion. If the thickness of the decarburized ferrite layer exceeds 5 μm, not only the fatigue characteristics of the steel material but also the impact characteristics are deteriorated. Therefore, the thickness of the decarburized ferrite layer is set to 5 μm or less.

Volume ratio of retained austenite: 10% to 40%
In the steel material according to the embodiment of the present invention, the volume ratio of retained austenite needs to be 10% or more in order to remarkably improve the ductility of the steel material while having a tensile strength of 980 MPa or more. On the other hand, if the volume fraction of retained austenite exceeds 40%, the delayed fracture resistance is deteriorated. For this reason, the volume ratio of retained austenite is made 40% or less.

The number density of cementite: less than 2 pieces / μm ^{2 In the} steel material according to the embodiment of the present invention, the number density of cementite is preferably less than 2 pieces / μm ^{2 in} order to significantly improve the impact characteristics. In addition, since it is better that the number density of cementite is small, there is no particular lower limit.

Average C concentration in retained austenite: 0.60% or less In addition, when the average C concentration in retained austenite is 0.60% or less by mass%, martensite generated with the TRIP phenomenon becomes soft, The occurrence of cracks is suppressed and the impact characteristics of the steel material are significantly improved. For this reason, the average C concentration in the retained austenite is preferably set to 0.60% or less in mass%. The lower the average C concentration in the retained austenite, the lower the better, so there is no particular lower limit.

3. Mechanical Properties The steel material according to the embodiment of the present invention has a tensile strength of 980 MPa or more. The tensile strength of the steel material is preferably 1000 MPa or more. Moreover, according to the steel material which concerns on embodiment of this invention, the outstanding ductility and impact characteristic can be acquired. For example, ductility with a product of tensile strength and total elongation of 16000 MPa ·% or more can be obtained. For example, it is possible to obtain an impact characteristic with an impact value of a Charpy test at 0 ° C. of 30 J / cm ² or more. Furthermore, when V is contained in the steel material, for example, a 0.2% yield strength (yield strength) with a yield strength of 900 MPa or more can be obtained.

4). Manufacturing method Although there is no restriction | limiting in particular about the manufacturing method of the steel materials which concern on this invention, For example, it can manufacture by giving the heat processing shown below with respect to the steel raw material which has said chemical composition.

4-1 Steel material As a steel material to be subjected to heat treatment, for example, a metal having a total volume ratio of bainite and martensite of 90% or more and an average aspect ratio of bainite and martensite of 1.5 or more. Use a tissue. The volume ratio of bainite and martensite is preferably 95% or more in total. Furthermore, when the V content of the steel material is 0.05% to 1.0%, it is preferable that 70% or more of the V contained in the steel material is in solid solution.

  When the volume fraction of bainite and martensite in the steel material is less than 90% in total, it is difficult to make the tensile strength of the steel material 980 MPa or more. Furthermore, there is a possibility that the volume ratio of retained austenite is lowered and ductility is deteriorated. Moreover, when the aspect ratio of bainite and martensite increases, cementite precipitates in parallel to the steel sheet surface, and decarburization is shielded. When the average aspect ratio of bainite and martensite is less than 1.5, the decarburization shielding is insufficient, and a decarburized ferrite layer is generated. Further, when the average aspect ratio of bainite and martensite is less than 1.5, nucleation of cementite is promoted and the cementite is finely dispersed, so that the number density is increased. The aspect ratio is a value obtained by dividing the major axis of each grain by the minor axis when observed from a section perpendicular to the rolling direction (hereinafter referred to as L section) with respect to the prior austenite grains of bainite and martensite. Further, an average value of aspect ratios obtained for all grains on the observation surface is adopted.

  Moreover, if V dissolved in V contained in steel is less than 70%, a desired yield strength cannot be obtained after heat treatment. Further, since the austenite growth during the heat treatment is delayed, the volume ratio of retained austenite may be lowered. Therefore, it is preferable that 70% or more of V contained in the steel material is dissolved. The solid solution amount of V can be measured, for example, by electrolytically extracting a steel material and then analyzing the residue using ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry).

  Said steel raw material can be manufactured by hot rolling of comparatively low temperature, for example. Specifically, hot rolling is performed so that the finishing temperature is 800 ° C. or less and the final pass reduction ratio is 10% or more, and the average cooling rate of 20 ° C./s or more is 600 ° C. or less within 3 s after finishing rolling. Quench quickly to the temperature of. Such relatively low temperature hot rolling is generally avoided because non-recrystallized grains are formed. When the steel material contains 0.05% or more of V, it is hot-rolled so that the finishing temperature is 950 ° C. or less and the reduction ratio of the final pass is 10% or more, and within 3 s after finishing rolling is finished. And rapidly cooling to a temperature of 600 ° C. or lower at an average cooling rate of 20 ° C./s or higher. Particularly when V is contained, the average aspect ratio of bainite and martensite tends to be 1.5 or more. Moreover, if the average value of the aspect ratio of bainite and martensite is a steel structure of 1.5 or more, the steel material may be tempered.

4-2 Heat Treatment As described above, the steel material according to the present invention can be manufactured by performing the following treatment on the steel material. Each step will be described in detail below.

a) Heating step First, the steel material is heated to a temperature of 670 ° C. or higher so that the average heating rate between 500 ° C. and 670 ° C. is 1 ° C./s to 5 ° C./s. Cementite has the effect of suppressing decarburization during heat treatment, but if coarse cementite remains in the steel material, the impact characteristics are significantly deteriorated. Therefore, control of the cementite particle size and the temperature between 500 ° C. and 670 ° C., which is easy to control the precipitation reaction, is extremely important.

  When the average heating rate is less than 1 ° C./s, cementite becomes coarse and decarburization is suppressed. However, coarse cementite remains in the steel after the heat treatment, and the impact characteristics deteriorate. Further, austenite is not sufficiently generated, and ductility may be deteriorated. On the other hand, when the average heating rate exceeds 5 ° C./s, cementite is easily dissolved during the heat treatment, and the decarburization reaction during the heat treatment cannot be suppressed.

  In heating to 500 ° C., the average heating rate is preferably 0.2 ° C./s to 500 ° C./s. When the average heating rate is lower than 0.2 ° C./s, the productivity is lowered. On the other hand, if the average heating rate exceeds 500 ° C./s, temperature control from 500 ° C. to 670 ° C. may be difficult due to overshoot or the like.

b) Holding step After the above heating, the holding step is held at a temperature range of 670 ° C to 780 ° C for 60s to 1200s. When the holding temperature is less than 670 ° C., not only the ductility deteriorates, but it may be difficult to make the tensile strength of the steel material 980 MPa or more. On the other hand, if the holding temperature exceeds 780 ° C., the volume ratio of the retained austenite of the steel material cannot be made 10% or more, and the ductility may be significantly deteriorated.

  Further, if the holding time is less than 60 s, the structure to be generated and the tensile strength are not stable, and thus it may be difficult to ensure a tensile strength of 980 MPa or more. On the other hand, when the holding time exceeds 1200 s, internal oxidation becomes remarkable, impact characteristics are deteriorated, and a decarburized ferrite layer is easily generated. The holding time is preferably 120 s or more, and preferably 900 s or less.

c) Cooling step After the heating and holding described above, cooling is performed to a temperature of 150 ° C. or lower so that the average cooling rate between the temperature range and 150 ° C. is 5 ° C./s to 500 ° C./s. When the average cooling rate is less than 5 ° C./s, soft ferrite and pearlite are excessively produced, and it may be difficult to make the steel material have a tensile strength of 980 MPa or more. On the other hand, if the average cooling rate exceeds 500 ° C./s, firing cracks are likely to occur.

  The average cooling rate is preferably 8 ° C./s or more, and preferably 100 ° C./s or less. If the average cooling rate up to 150 ° C. is 5 ° C./s to 500 ° C./s, the cooling rate at 150 ° C. or less may be the same as or different from the above range.

  Further, C tends to be unevenly distributed in austenite in the temperature range from 350 ° C. to 150 ° C. during cooling. Therefore, in order to set the average C concentration in the retained austenite of the steel material to 0.60% or less, it is preferable to cool so that the residence time in the temperature range is 40 s or less.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steel materials having the chemical composition shown in Table 1 and the metal structure shown in Table 2 were subjected to heat treatment under the conditions shown in Table 3.

  The steel material used was manufactured by hot working a slab melted in the laboratory under the conditions shown in Table 2. This steel material was cut into dimensions of 1.6 mm in thickness, 100 mm in width, and 200 mm in length, and heated, held and cooled according to the conditions in Table 3. A thermocouple was attached to the surface of the steel material, and the temperature was measured during the heat treatment. The average heating rate shown in Table 3 is a value between 500 ° C. and 670 ° C., and the holding time is the time held at that temperature after reaching the holding temperature. The average cooling rate is a value between the holding temperature and 150 ° C., and the residence time is a residence time in the temperature range from 350 ° C. to 150 ° C. during cooling.

  As explained below, the metal structure of the steel material before heat treatment, the metal structure and mechanical properties of the steel material obtained by heat treatment were investigated by metal structure observation, X-ray diffraction measurement, tensile test, and Charpy impact test. .

<Metal structure of steel material>
The L section of the steel material was observed and photographed with an electron microscope, and the area ratio and aspect ratio of bainite and martensite were measured by analyzing a total area of 0.04 mm ² . And since the structure | tissue of a steel raw material is isotropic, the value of the said area ratio was made into the volume ratio of a bainite and a martensite. The aspect ratio was determined by dividing the major axis of each grain by the minor axis with respect to the prior austenite grains of bainite and martensite, and the average value was calculated.

  The observation position was set to a position of about 1/4 of the plate thickness (a position of 1/4 t), avoiding the center segregation portion. The reason for avoiding the center segregation part is as follows. The center segregation part may have a locally different metal structure with respect to a typical metal structure of a steel material. However, the center segregation portion is a small region with respect to the entire plate thickness, and hardly affects the characteristics of the steel material. That is, it cannot be said that the metal structure of the central segregation part represents the metal structure of the steel material. Therefore, it is preferable to avoid the center segregation part in the identification of the metal structure.

<Solution V amount of steel material>
After electrolytic extraction of the steel material, the residue was analyzed using ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry), thereby measuring the amount of V dissolved in the steel material.

<Metal structure of steel>
A test piece having a width of 20 mm and a length of 20 mm was taken from each steel material, and this test piece was subjected to chemical polishing to reduce the thickness by 0.4 mm, and X-ray diffraction was performed three times on the surface of the test piece after chemical polishing. did. The obtained profiles were analyzed, and each was averaged to calculate the volume fraction of retained austenite.

<Average C concentration in retained austenite>
The profile obtained by X-ray diffraction was analyzed, the lattice constant of austenite was calculated, and the average C concentration in retained austenite was determined based on the following equation.
c = (a−3.572) /0.033
However, the meaning of each symbol in the above formula is as follows.
a: Lattice constant of austenite (Å)
c: Average C concentration (mass%) in retained austenite

<Thickness of decarburized ferrite layer>
The thickness of the decarburized ferrite layer was measured by observing and photographing the L cross section of the steel material with an electron microscope and analyzing the 1 mm region on the steel plate surface.

<Cementite number density>
Regarding the number density of cementite, the number density of cementite was measured by analyzing a total area of 2500 μm ² .

<Tensile test>
A JIS No. 5 tensile test piece having a thickness of 1.6 mm was collected from each steel material, and subjected to a tensile test according to JIS Z 2241 (2011). TS (tensile strength), YS (yield strength, 0.2% proof stress) and EL (total elongation) was measured. The value of TS × EL was calculated from TS and EL.

<Impact characteristics>
The front and back surfaces of each steel material were ground to a thickness of 1.2 mm to produce V-notch test pieces. Four test pieces were stacked and screwed, and then subjected to a Charpy impact test according to JIS Z 2242 (2005). As for impact characteristics, a case where the impact value at 0 ° C. was 30 J / cm ² or more was judged as good (◯), and a case where it was less than 30 J / cm ^{2 was} judged as poor (×).

  The results of observation of the metal structure of the steel material are summarized in Table 2, and the results of X-ray diffraction measurement, tensile test and Charpy impact test are summarized in Table 4.

  As shown in Tables 2 to 4, test numbers 2, 4, 9, 34, and 44, which are comparative examples, had an aspect ratio of steel material of bainite and martensite of less than 1.5. The thickness exceeded 5 μm, and as a result, the impact characteristics were poor. In Test Nos. 8 and 39, since the average cooling rate was low, barlite was generated excessively, and a tensile strength of 980 MPa or more was not obtained. In Test No. 3, due to the high average heating rate in the heat treatment, the thickness of the decarburized ferrite layer was 5 μm or more, and as a result, the impact characteristics were poor.

  Test No. 11 was inferior in impact characteristics because the Si content was higher than the specified range. Test No. 14 had inferior impact characteristics because the C content was higher than the specified range. In Test Nos. 13 and 32, since the holding temperature in the heat treatment was high, the volume ratio of retained austenite was low, and as a result, the ductility was poor. In Test No. 17, since the holding time in the heat treatment was long, the thickness of the decarburized ferrite layer was 5 μm or more, and as a result, the impact characteristics were poor.

  Test Nos. 18 and 26 had a Mn content lower than the specified range, Test No. 24 had a C content lower than the specified range, and Test No. 29 had a Si content lower than the specified range. Not only was it bad, but a tensile strength of 980 MPa or higher was not obtained. In Test No. 23, since the heating rate in the heat treatment was low, the volume ratio of the retained austenite was low, and as a result, the ductility was deteriorated and the impact characteristics were also poor. In Test No. 31, since the holding time in the heat treatment was short, the structure to be generated and the tensile strength were not stable, and a tensile strength of 980 MPa or more was not obtained. In Test No. 40, the volume fractions of bainite and martensite were less than 90% in total, so in Test No. 43, the retention temperature in the heat treatment was low, so the volume fraction of retained austenite was low, resulting in poor ductility. Furthermore, a tensile strength of 980 MPa or more was not obtained.

On the other hand, test numbers 1, 5-7, 10, 12, 15, 16, 19-22, 25, 27, 28, 30, 33, 35-38, 41, 42 and 45-47, which are examples of the present invention, While having a tensile strength of 980 MPa or more, the product of the tensile strength and the total elongation (TS × EL) has an excellent ductility of 16000 MPa ·% or more, and has an impact value of 30 J / cm ² at 0 ° C. The impact characteristics were also good.

  According to the present invention, it can be used for, for example, an automobile-related industry, an energy-related industry, and an architecture-related industry.

Claims (7)

% By mass
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: more than 3.0% and 7.5% or less,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%
Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1.0%
Mo: 0% to 1.0%
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0.01%
Zr: 0% to 0.01%,
B: 0% to 0.01%
Bi: 0% to 0.01%, and the balance: Fe and impurities,
Having a chemical composition represented by
The decarburized ferrite layer has a metal structure having a thickness of 5 μm or less, a volume ratio of retained austenite of 10% to 40%, and a number density of cementite of less than 2 pieces / μm ² ,
A steel material having a tensile strength of 980 MPa or more. In the chemical composition,
V: 0.05% to 1.0%
The steel material according to claim 1 , wherein: In the chemical composition,
Ti: 0.003% to 1.0%,
Nb: 0.003% to 1.0%,
Cr: 0.01% to 1.0%
Mo: 0.01% to 1.0%
Cu: 0.01% to 1.0%, or Ni: 0.01% to 1.0%,
Or the arbitrary combination of these is satisfy | filled, The steel materials of Claim 1 or 2 characterized by the above-mentioned. In the chemical composition,
Ca: 0.0003% to 0.01%,
Mg: 0.0003% to 0.01%
REM: 0.0003% to 0.01%,
Zr: 0.0003% to 0.01%,
B: 0.0003% to 0.01%, or Bi: 0.0003% to 0.01%,
Alternatively, the steel material according to any one of claims 1 to 3 , wherein any combination thereof is satisfied. The average C concentration in the residual austenite, in mass%, the steel according to any one of claims 1 4, characterized in that at 0.60%. A method for producing a steel material according to any one of claims 1 to 5,
% By mass
C: 0.050% to 0.35%,
Si: 0.50% to 3.0%,
Mn: more than 3.0% and 7.5% or less,
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 0.001% to 3.0%,
N: 0.01% or less,
V: 0% to 1.0%
Ti: 0% to 1.0%
Nb: 0% to 1.0%
Cr: 0% to 1.0%
Mo: 0% to 1.0%
Cu: 0% to 1.0%,
Ni: 0% to 1.0%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
REM: 0% to 0.01%
Zr: 0% to 0.01%,
B: 0% to 0.01%
Bi: 0% to 0.01%, and the balance: Fe and impurities,
Steel having a metal structure in which the volume ratio of bainite and martensite is 90% or more in total, and the average aspect ratio of bainite and martensite is 1.5 or more. Heating the material to a temperature of 670 ° C. or higher so that the average heating rate between 500 ° C. and 670 ° C. is 1 ° C./s to 5 ° C./s;
After the heating, holding for 60 s to 1200 s in a temperature range of 670 ° C. to 780 ° C.,
And a step of cooling to a temperature of 150 ° C. or lower so that an average cooling rate between the temperature range and 150 ° C. is 5 ° C./s to 500 ° C./s after the holding. Manufacturing method. In the chemical composition,
V: 0.05% to 1.0%
Is satisfied,
The method for manufacturing a steel material according to claim 6 , wherein 70% or more of V contained in the steel material is dissolved.